If an imaging optical system for infrared imaging devices is realized by transmission optical elements such as lenses, it is necessary to use germanium as a raw material and the price rises.
Accordingly, to realize a low-price imaging optical system, an imaging optical system using reflecting mirrors is preferable. An imaging optical system using reflecting mirrors is disclosed in JP2004-126510, for example.
However, the imaging optical system described above is not bright enough to be used for infrared imaging devices such as infrared cameras. An imaging optical system using reflecting mirrors which is compact enough to be mounted on vehicles or the like and is sufficiently bright has not been developed.
In order to achieve an object of the present invention, brightness (F-number, Fno.) of an imaging optical system used for infrared and far-infrared lights (7 to 14 micrometers) should be 2.2 or less (brighter) to prevent deterioration in resolution due to effects of diffraction. When F-number is greater than the value, the beam spot will be larger than the pixel size of the commercially available infrared imaging element and therefore the optical system cannot meet requirement of resolution of the infrared imaging element. For visible lights F-number of 6 or less is sufficient. So, imaging optical systems used for visible lights can hardly be used for far-infrared lights.
Further, in such optical systems using reflection as that of the present invention, stray lights often reach the image plane, and therefore to study how to remove stray lights is more important than to study how to correct aberration. On the contrary, in optical systems using refraction it is almost unnecessary to study effects of stray lights.
That is, in order to achieve the object of the present invention, it is necessary to study how to obtain F number of 2.2 or less and how to remove stray lights, which has not been studied in the prior art and therefore cannot be studied based on the prior art.
Accordingly, there is a need for an imaging optical system using reflecting mirrors which is compact enough to be mounted on vehicles or the like and is sufficiently bright and from which stray lights have been sufficiently removed.